The risk of bird flu evolving into a more transmissible agent in humans remains high, based on the widespread distribution of the H5N1 virus in poultry and the continued exposure of humans, according to the United Nations health agency’s first analysis of epidemiological data on all cases reported to it from December 2003 to April 2006.
The analysis by the World Health Organization (WHO) reports a mortality rate of over 50 per cent and calls for better collection of essential data to understand and refine case management of H5N1, which experts fear could, in a worst case scenario, mutate into a deadly human pandemic.
“The sharing of data may be seen as part of an early warning system that will collectively defend all countries against a common threat,” it says, noting that in May the UN World Health Assembly called for immediate voluntary compliance with provisions seeking such speedy communications.
“If countries comply with these provisions, they will greatly assist themselves, the international community and WHO in monitoring evolving situations and supporting adequate responses as well as enabling reliable risk assessments to be made,” it adds.
As of the end of last month there had been only 228 confirmed human cases, 130 of them fatal, since the current outbreak started in South East Asia in December 2003, nearly all of them ascribed to contact with infected birds. The so-called Spanish flu outbreak of 1918, also starting from a bird flu virus, is estimated to have killed from 20 million to 40 million people worldwide by the time it run its course two years later.
The analysis, based on epidemiological data on all 205 laboratory-confirmed cases reported to WHO through the end of this April, reaches the following conclusions:
- The overall case-fatality rate was 56 per cent, with the highest rate in persons aged 10 to 39 years.
- Half of the cases occurred in people under the age of 20 and 90 per cent of cases in people under 40 years.
- The case-fatality profile by age group differed from that seen in seasonal influenza, where mortality is highest in the elderly.
- The number of new countries reporting human cases increased from 4 to 9 after last October following the geographical extension of outbreaks among birds.
- The overall case-fatality rate was highest in 2004 (73 per cent), followed by 63 per cent to date in 2006, and 43 per cent in 2005.
- Cases have occurred all year round, but human cases peaked during the period roughly corresponding to winter and spring in the northern hemisphere. If this pattern continues, an upsurge in cases could be anticipated starting in late 2006 or early 2007.
- The median interval between symptom onset and hospitalization and symptom onset and death remained substantially unchanged during the three years, at 4 days for the former 9 days for the latter.
As of 20 June, 10 countries had reported human cases: Azerbaijan, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Thailand, Turkey and Viet Nam. Viet Nam has registered the highest number of cases and deaths at 93 and 42 respectively, though none this year, followed by Indonesia with 51 and 39, most of them this year.